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Material selection and mechanical design of submerged galvanising bath journal bearings

Faulkner, Rhys 2020. Material selection and mechanical design of submerged galvanising bath journal bearings. EngD Thesis, Cardiff University.
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Material selection, operating conditions and changes to the geometric design of journal bearings submerged in an automotive grade molten Zn galvanising bath with 0.4 wt% Al have been investigated. The goal of this research was to promote stable operation of galvanising pot hardware through increased stability and service life of submerged bearings, capable of producing high quality ‘full-finish’ automotive grade galvanised ‘GI’ strip steel for extended periods of time. Several suitable materials were identified for use in the molten zinc bath, including three cast Co-based superalloy materials of Wallex 6, Wallex 4 and Wallex 180. Four laser clad materials, two Co-based grades of Wallex 6 and T400, and two Fe-based grades of D2 Tool Steel and Colferoloy 103 were selected for analysis. Samples of 25 mm diameter and 25 mm length were produced for each material and analysed. Several discrepancies were noted between cast and laser clad microstructures, including dissimilar phase compositions and phase dispersions present in the microstructure, particularly between equivalent Wallex 6 cast and laser clad materials and T400 and Wallex 180 of equivalent chemical composition. In general laser clad materials were noted to have refined microstructures, owing to the poor optimisation of the laser cladding process. A study of submerged galvanising bath journal bearing operating conditions was undertaken and identified a large variation between the loads of the sink roll and stabiliser roll bearings used in the continuous galvanising line. The sink roll bearings are subject to load nearly three times greater than the stabiliser roll bearings. The maximum sink roll bearing load was calculated to be approximately 35,500 kN compared to 15,500 kN for the stabiliser roll bearing. The speed of the two rolls, and attached bearings, was also calculated to differ substantially, with the rotational speed of the sink roll four times slower than the stabiliser roll. Average rotational speed of the sink roll was calculated to be 48 RPM whilst the average rotational speed of the stabiliser roll was determined to be 195 RPM. Line speed and tension for the Zodiac galvanising line were plotted for an entire five-week galvanising line and were observed to fluctuate corresponding to product schedule. Operating conditions become more erratic when general purpose ‘GP’ grade product is being processed. Hydrodynamic bearing feasibility for galvanising bath applications was conducted using a two-dimensional Reynolds equation solved with a finite difference code written in MATLAB. The solution of Reynolds equation showed that current bearings are operating within the mixed or boundary lubrication regime with negligible load capacity developed. 3 However, increasing the bearing sizes and reducing clearance showed that hydrodynamic lubrication of journal bearings is theoretically possible in the molten Zn galvanising bath. The selected materials were submerged in molten Zn and 0.4 wt% Al for time intervals of up to five weeks. All materials reacted with molten Zn and Al, and the laser clad materials were noted to perform worse than equivalent cast materials except for laser clad T400. Poor performance of laser clad materials was deemed to be a result poor optimisation of the laser clad coatings. The best material in terms of static immersion performance was laser clad T400 which displayed limited evidence of reaction with the molten Zn and Al relative to other materials. This was quantified when the materials were pickled in 35% conc. HCl with surface roughness profiles taken to assess surface damage. Laser clad T400 performed exceptionally well in this test relative to the other materials, corroborated the limited reactions observed between the microstructure and molten Zn and Al. In addition to static immersion testing work, a bespoke dynamic bearing testing rig was developed which permits offline testing of full-scale journal bearings. The bearing rig can simulate accurate bearing load and rotational speeds whilst submerging bearings in a crucible of molten Zn and Al. The machine structure was simulated with FEA and determined to be well designed to meet operational stresses.

Item Type: Thesis (EngD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Tribology; Journal Bearings; Galvanising; Galvanising Hardware; Steelite; Cobalt Super Alloy.
Date of First Compliant Deposit: 20 August 2020
Last Modified: 25 Jun 2021 14:18

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